120 DESCRIPTIVE BOTANY. PART I. 



afforded them by the pruning knife, that they are 

 enabled to grow. Sometimes the development of an 

 organ is impeded or prevented, by the want of a suffi- 

 cient supply of nutriment ; and this often arises from 

 the abstraction of what was naturally destined for it, 

 by the more vigorous growth of some neighbouring 

 portion. Hence the different characters which dis- 

 tinct individuals of the same species assume, depend 

 upon the various degrees of influence which those and 

 many other external circumstances have upon them. 

 From such causes as these, we find the leaves of a 

 tree gradually dwindling into membranous scales ; the 

 calyx of the florets in the Composite becoming a 

 downy pappus (fig. 11?.). The thorny prickles of 

 the wild plum are merely stunted branches, and by 

 culture readily disappear, an effect which Linnieus 

 fancifully termed, the taming of wild fruits. But 

 besides these merely external influences, which may all 

 be considered as accidental causes, tending to produce 

 the abortion of particular parts, there are others of a 

 more subtle and incomprehensible description, which 

 are in constant operation within the plant ; and which, 

 acting from the very earliest periods in which certain 

 organs begin to develop, tend to suppress or modify 

 them; and thus produce that infinite diversity of 

 forms and characters, which we find even among those 

 which are destined to perform the very same function. 

 And sometimes the altered organs are so far changed 

 from their original character, as to become adapted only 

 to serve some new secondary purpose, distinct from 

 that for which they were primarily intended. Thus, 

 the spines of the common furze (Ulex europceus), are 

 merely modified leaves. In the common berberry 

 (Herheris vulgaris}, the transition may be readily traced 

 (see fig. 68.). 



(118.) Adhesion. If to the operation of the two 

 causes already noticed, we add the " adhesions," which 

 take place between the contiguous parts of similar or 

 different organs, we introduce a third cause, in very 



